Alien movement patterns

Closely linked to Spawn Points are the alien movement patterns (aka routes). Although the maps are randomly generated, each module (especially ones with doorways) have patrol points defined. Aliens on patrol will follow a route through these points until they spot the enemy (you). Then the aggression formula takes over.

Guards

There's a safe method to dealing with almost every UFO. Due to odd positioning, some ship have alien 'guards' that do not move from their spot until you 'trigger' them somehow, either by walking through a door or standing at certain locations in the UFO. In general, they'll head in the general direction of the nearest soldier to them.

[ Hobbes: This is debatable - When the AI is moving an alien it can happen that the next waypoint is already occupied by another alien unit and thus it will freeze on its location without moving. In those cases what will happen is that the alien will wait until the path is clear before moving.]

For example: The Large Scout occasionally has one nasty one, depending on the exact location where the alien spawns in the bridge. When approaching the bridge, head to the left passage (your soldier's relative left), rather than the more obvious one to the right (relative o the screen, this is the upper passage)- where you often get killed instantly the moment you step through that door.

Wait at the corner just before the left door and the alien will start moving towards you. By the next turn it should be in the room just beyond -- exhausted and unable to fight back.

Other UFOs may require bigger weaponry to deal with these 'guards'. The Abductor for example has a guard that always waits in the centre of the bridge. It's untested, but it may be possible to get this guard to face in a different direction by placing a soldier as close as possible to it and ending the turn. On the next turn, have a soldier barge through the door. Again, this is untested, so use it at your own risk. More practical solutions to this include cutting a hole through the rear or floor of the room with a Heavy Plasma, deploying a smoke grenade in front of the door, stepping through and launching a rocket or a grenade, or even just standing underneath the alien and firing a high explosive weapon directly at the ceiling.

Dislodging Guards

Recent testing has suggested that aliens which remain fixed in place are stuck in "aggression mode" against one X-COM unit. As long as that unit does not move, the alien will not move either. To flush aliens out of fixed positions, try moving all your units, one at a time, and keep an ambush posted outside the UFO in case the alien emerges.

Floaters and the Koan of Stairs

How did those Floaters get up those stairs in the first place? They certainly don't know how to go down them.

Eventually, the hapless floater will have the AI try to route it from the top of a staircase to the bottom of the same staircase. Unfortunately, not having legs, the art of walking down stairs escapes the floater. With a few precautions to abuse line-of-fire bugs, this makes the stuck Floater easy prey for even a lone X-COM agent.